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Bosch · Boiler

E3 Additional supply temperature sensor fault

Medium severity Bosch Greenstar

What the control is telling you

The extra supply temperature sensor reads as faulty.

Applies to: Bosch Greenstar ZBR 21/28/35/42-3A boilers and ZWB 28/35/42-3A combi boilers

Likely causes

  • Open or shorted sensor or lead

Test and repair steps

  1. Warning: Disconnect mains power and close the gas cock before opening the appliance or working on the gas train.
  2. Check the sensor and its leads; replace if defective.

Models that show this code

4 models in Bosch Greenstar

ZBR21-3AZBR28-3AZBR35-3AZBR42-3A

Other codes on the same units

  • C6 Fan not running
  • d3 High limit / temperature limiter circuit open
  • d5 External supply temperature sensor fault (low-loss header)
  • E2 Supply temperature sensor fault
  • E9 Safety high limit or flue gas limiter tripped
  • F0 Internal fault
  • F1 Internal data error
  • F7 Flame detected with burner off

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Source: Bosch Greenstar ZBR/ZWB 21-42-3A service manual 6721853955 (2025-11), Table 38 Faults shown on the display, p.74-75; Greenstar combi 100 p/151 p service manual 6721853953 (2025-11), Table 33. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.